In a preferred form of the invention described in the parent applications, a superconductor is produced by the steps of combining a plurality of layers of metal sheets to form a composite structure. The sheets are preferably pure transition metals--niobium, titanium, zirconium, or vanadium, for example--alternate sheets being formed of different transition metals. The resulting composite structure is mechanically reduced sufficiently so that each transition metal sheet is less than 1000 A.degree. thick. In the course of reduction, the composite is subjected to sufficient temperatures for sufficient times such that the transition metal layers are partially reacted to form a ductile superconducting material between the transition metal layers.
Approximately one half by volume of the transition metal layers remain unreacted. These unreacted layers afford efficient flux pinning within the composite when the layers are reduced to the &lt;1000 A.degree. final size. In the fabrication of ternary (or higher order) alloys like NbTiTa, one or more of the transition metal layers is made relatively thin so as to allow complete diffusion through that region. However, at least one half by volume of one of the constituent transition metal layers remains pure or nearly pure after the reaction so as to provide pinning within the ternary composite when reduced to &lt;1000 A.degree. in thickness. In other embodiments, powders and filaments can be used instead of initial layers.
The present invention relates to the fabrication of wire containing layers of A15-type superconducting material, Nb.sub.3 Sn and Nb.sub.3 Al, e.g. U.S. Pat. No. 3,218,693, held by National Research Coporation, describes a method for fabricating Nb.sub.3 Sn conductors by assembling layers of niobium and tin, mechanically reducing the layer thicknesses by at least 98%, and then heating the product to form Nb.sub.3 Sn layers. Excess Nb remains to provide mechanical support for the Nb.sub.3 Sn. While similar in many ways, the present invention involves much greater layer reduction than is called for in the NRC patent, and the superconducting material is formed during hot processing, not during a separate heat treatment. It should also be noted that the NRC inventors were clearly unaware of the flux pinning that normal/superconducting interfaces will provide when the layers of normal and superconducting material are &lt;1000 A.degree. thick. This principle is at the heart of the present invention.
The efficacy of artificial pinning for the A15 compound Nb.sub.3 Sn was recently demonstrated by Gauss et al. (see, "Nb.sub.3 Sn Multifilamentary Wires with composite Core Filaments", S. Gauss, W. Specking, F. Weiss, E. Seibt, J. Xu, and R. Flukiger, Advances in Cryogenic Engineering (Materials), vol 34, p. 843-849, Edited by A. F. Clark and R. P. Reed, Plenum Press, New York, 1988). This research showed that the incorporation of tantalum, titanium, or NbTi as pinning material in Nb.sub.3 Sn can result in substantially higher Nb.sub.3 Sn current density (J.sub.c) as compared to conventionally fabricated "bronze-route" Nb.sub.3 Sn, where only grain boundaries are available as pinning sites. The composites produced by Gauss et al. were entirely unlike the layered composites typical of the present invention, but the same mechanisms are at work in both cases.
Nb.sub.3 Al superconducting wires have been fabricated by a jellyroll method similar to that utilized in the application of the present invention (see, for example, "Experimental Result on Nb.sub.3 Al Multifilamentary Wires", R. Bruzzese, N. Sacchetti, M. Spadoni, G. Barani, S. Ceresara, G. Donati, Proceedings of the Ninth International Conference on Magnet Technology, p. 557-559, Edited by C. Marinucci and P. Weymouth, Swiss Institute for Nuclear Research, Zurich, 1985). As in the case of the NRC Nb.sub.3 Sn processing, the Nb.sub.3 Al is formed in the composites during a special heat treatment given at the final wire size. It is one of the primary stengths of the present invention that such heat treatments are not required and, indeed, can result in poor J.sub.c due to degradation of the pinning layer purity.